Molecular Dynamics Investigation of the Potentiality of Zeolite NaY for Trimethylbenzene Isomer Separation: An Insight in Terms of Structure, Energetics, and Dynamics.

Isomers of trimethylbenzene (TMB) are the important constituent chemicals used in a variety of industrial applications, including the production of synthetic resins, insecticides, dyes, pigments, and pharmaceuticals. However, these applications require them in their purest form, mandating them to separate from their mixtures. Due to the similar physicochemical properties of the TMB isomers, traditional methods such as cryogenic distillation are very energy-expensive. Thus, adsorption-based separation methods using nanoporous adsorbents such as zeolite are a cost-effective alternate. Such adsorption-based separation methods, however, require the isomers to exhibit significant differences in their adsorption and diffusion properties. In the present report, we carried out a thorough investigation of the structural, energetic, and dynamical properties of the three isomers of TMB in a faujasite-type zeolite NaY containing cage-like pores. Our results indicate that 1,2,4-TMB exhibits the most facile translational and out-of-plane rotational motions in contrast to the other two isomers. However, the in-plane rotational motions are seen to be more facile in 1,2,3-TMB compared with the other two isomers. It is evident from our analysis that the dynamics are dominantly driven by entropy. These results highlight the importance of the porous material in the case of the separation of a given hydrocarbon mixture.

Thermodynamics of translational and rotational dynamics of C9 hydrocarbons in the pores of zeolite-beta.

There has been a growing interest in the separation of aromatic hydrocarbon molecules from the petroleum stream using zeolite-based technologies. This led to numerous experimental and molecular simulation studies of the structural and dynamical properties of aromatic hydrocarbons under the confinement of microporous materials like zeolites. The understanding of the behavior of the isomers of the trimethylbenzene under confinement is crucial for their separation and purification from industrial streams. Here, we investigate the translational and rotational dynamics and associated thermodynamics of three isomers of trimethyl benzene, namely, 1,2,3-trimethyl benzene (1,2,3-TMB), 1,2,4-trimethyl benzene (1,2,4-TMB), and 1,3,5-trimethylbenzene (1,3,5-TMB) under the confinement of zeolite-beta (BEA) using molecular dynamics (MD) simulations. The trends in the diffusion coefficients of the TMB isomers calculated from our MD simulation data are in good agreement with the data already available in the literature. Analysis of dynamics and associated thermodynamic properties indicate that 1,2,4-TMB is translationally more facile than the other two isomers. The rotational motion of TMB isomers is largely anisotropic and it is relatively more significant for both 1,2,4-TMB and 1,3,5-TMB. The thermodynamic properties reveal that the distinguishability in the dynamic properties among these three isomers is essentially caused by entropy. These results are not only critical to engineer the separation process of TMB isomers across the zeolite beds but also to understand the different catalytic processes such as trans-alkylation, conversion, cracking etc.

In-silico identification of adsorbent for separation of ethane/ethylene mixture.

We present here a high-throughput computational screening of 4,821 real metal-organic framework (MOF) structures that do not contain any open metal sites to isolate the best performing candidate for separation of ethane/ethylene mixture at ambient conditions. The MOF structures were assessed on the basis of several adsorption-based separation performance metrics. Some of these metrics were found to correlate strongly among themselves. We have presented various structures-property correlations which unfold useful insights. MOF ATAGEJ is found to be the top performing MOF with highest adsorbent performance score 12.38 mol/kg and regenerability 93.88%. Several other MOFs OTOLIU (MIL-167), UMUMOG (UBMOF-8), and TOVGES (PCN-230) containing tetravalent metal cations such as Zr4+ and Ti4+ are found to be potential structures that are thermally, mechanically, and chemically stable and performs better than zeolites. Adsorption selectivity shows exponential correlation with difference of heat of adsorption of ethane and ethene at 0.1 bar and 298 K. We have also presented how various performance metrics correlate among themselves. These correlations unfold useful insights. Graphical abstract.

Separation of methane from ethane and propane by selective adsorption and diffusion in MOF Cu-BTC: A molecular simulation study.

Pure methane is an alternative source of cleaner energy. Although, natural gas contains around 90% of methane, there are other heavier alkanes such as ethane and propane. Presence of these heavier hydrocarbons affects the reusability of an adsorbed natural gas system (ANG). Thus, separation of these higher alkanes from methane is important. In the present study, we employed molecular simulation techniques to assess the performance of MOF Cu-BTC for separation of methane from ethane and propane at 298 K and for a range of pressure. The assessment was carried out on the basis of variety of performance metrics suitable for adsorption based separation. The performance metrics that we relied upon are adsorption selectivity, working capacity, regenerability (R%), adsorption performance score (APS), diffusion selectivity, and membrane selectivity. We investigated the performance for two equimolar binary mixtures - methane/ethane and methane/propane, and two ternary mixtures - one equimolar mixture and the other containing 90% methane, 7% ethane, and 3% propane. The adsorption selectivity of ethane over methane and propane over methane are really attractive indicating good performance of the MOF in separating the two binary mixtures. We also investigated the effect of the presence of ethane and propane on the mobility of methane. The diffusivities of methane, although, reduces by some factor in presence of ethane and propane, are sufficiently higher at all the pressures and at different compositions with ethane and propane. Finally, we assessed the performance of Cu-BTC as a membrane for separation of methane from ethane and propane.